HVAC actuators are used in a wide variety of HVAC systems and applications. Such actuators can include, for example, air flow damper actuators, water valves, gas valves, as well as other actuators. In many cases, a motor is used to move the actuator, and a controller is used to provide control signals to cause the motor to drive the actuator to a desired actuated position.
HVAC actuator controllers produced today typically either provide analog output control signals or “floating” binary output control signals to control the position of the actuator. Analog output control signals are conventionally either 0-10 volt signals or 4-20 milliamp signals. HVAC actuator controllers that use analog output control signals must typically include a relatively expensive and accurate digital-to-analog (DA) converter. Furthermore, and during use in the field, analog output control signals can often be susceptible to electromagnetic noise that can affect the accuracy and/or reliability of the actuator control. Also, and at least in some cases, there may be a time lag due to the D/A conversion, which can effect the accuracy and/or reliability of the actuator control.
“Floating” binary output control signals can have some advantages over analog output control signals. For example, HVAC actuator controllers that provide floating binary output control signals do not typically require a digital-to-analog (DA) converter, are often less susceptible to electromagnetic noise, and are typically not subject to a time lag due to a D/A conversion. In a typical HVAC actuator that uses “floating” binary output control signals, the HVAC actuator controller provides two separate floating binary output control signals, each for commanding the actuated part to move in a particular direction (e.g. open or close). The term “floating” is used here to signify that the separate binary output control signals provide “relative position” control commands, rather than “absolute position” control commands to move the actuated part.
In one example one of the floating binary output control signals may be asserted high to move the damper actuator from whatever its current position is towards the “closed” position, while the other of the floating binary output control signals may be asserted to move the damper actuator from whatever its current position is towards the “open” position. Only one of the two floating binary output control signals is typically asserted at any given time.
In many cases, the amount or degree that the damper actuator is moved from its current position is dependent on the time interval that the floating binary output control signal is in the asserted state (e.g. high state). For example, if the controller asserts the floating binary output control signal that moves the damper actuator towards the “closed” position for 50 ms, the motor may move the damper actuator 2 degrees from its current position toward the “closed” position. As can be seen, and in this arrangement, the floating binary output control signals move the damper actuator relative to whatever its current position is, rather than commanding an absolute damper position (e.g. 60% open). The HVAC actuator controller must typically include software and/or hardware to indirectly keep track of the current position of the damper actuator, and make relatively position changes based on the desired position of the damper.
In today's market, the cost of an HVAC controller is typically directly related to the number of outputs on the controller. Even though floating binary outputs are cheaper to produce than analog outputs, a floating binary output controller still requires at least two separate outputs per HVAC actuator. What would be desirable, therefore, is a method and system for controlling an HVAC actuator that maintains the advantages of floating binary outputs, which includes high resolution with high noise immunity and lower cost, but requiring less controller outputs.